Apparatuses employing an organic electrolumimescence device (organic EL device) have been developed in recent years. For example, these apparatuses are formed by providing, on a substrate of glass or the like, an anode, an organic layer (such as a hole-transporting layer, a luminescent layer and an electron-transporting layer), a cathode, or the like, and connecting the same to an external wiring via wirings (terminals) of the two electrodes. When a voltage is applied between the electrodes, excitons are generated in the luminescent layer positioned between the electrodes, as a result of re-combination of holes and electrons, and excessive energy is emitted as light when the excitons return from the excited state to the ground state.
The apparatuses employing the organic EL device as mentioned above can reduce the thickness thereof as compared with display apparatuses employing a liquid crystal or the like. Moreover, when the apparatuses emit white light, these can be applied to a backlight for a liquid crystal display or a illumination device, in addition to a display for a television or a cellular phone.
In order to emit white light using an organic EL device, a method of forming three luminescent layers each emit light of red (R), green (G) and blue (B), respectively, and a method of dispersing three kinds of luminescent materials for RGB in a single luminescent layer, have been known.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2001-319780 proposes a luminescent device having a luminescent layer that includes at least two kinds of luminescent materials, and at least one of these luminescent materials is an ortho-metallized complex, in order to improve luminescence efficiency or brightness as a white light source.
JP-A No. 2002-100476 proposes a luminescent device having a luminescent layer that includes a luminescent material and a host material, the luminescent material having a maximum luminescence peak wavelength of 500 nm, and the host material having a lowest excited triplet energy level that is higher than that of the luminescent material, in order to improve luminescent efficiency and color purity in emitting white light.
JP-A No. 2007-27092 proposes an organic EL device that emits white light, the device having a blue luminescent layer and a mixed luminescent layer of green and red, the mixed luminescent layer including a hole-transporting material, an electron-transporting material, and a phosphorescent material.